1. Field of the Invention
The present invention relates to an apparatus and a method for calibrating signals, and more particularly, to an apparatus and a method for calibrating mismatch between in-phase and quadrature-phase signals.
2. Description of the Prior Art
FIG. 1 is a diagram showing a conventional zero-IF receiver 10. The zero-IF receiver 10 includes an antenna 11, a low noise amplifier (LNA) 12, mixers 14 and 24, low pass filters (LPFs) 16 and 26, analog to digital converters (ADCs) 18 and 28, and a digital signal processor (DSP) 19. Antenna 11 receives a radio signal, and LNA 12 amplifies the radio signal. Mixer 14 mixes the radio signal with a first carrier wave(COSωct in FIG. 1), so as to generate an analog signal Sa1. The other mixer 24 mixes the radio signal with a second carrier wave (SIN(ωct+δ) in FIG. 1), so as to generate an analog signal Sa2. LPFs 16,26 filter out high frequency components of the analog signals Sa1, Sa2, respectively. Additionally, ADCs 18,28 convert the analog signals Sa1 and Sa2 to corresponding digital signals Sd1 and Sd2, respectively, and DSP 19 further processes the digital signals Sd1 and Sd2.
Ideally, there should be a 90° phase difference between the above-mentioned first carrier wave and the second carrier wave for generating analog signals Sa1 and Sa2 with quadrature relation (i.e., an in-phase signal and a quadrature-phase signal). However, in actual apparatus, temperature variation, process variation, and drift of supplied power may cause a phase offset δ between the first carrier wave and the second carrier wave such that the phase difference can't be ideal. This problem is called IQ mismatch.
IQ mismatch influences demodulation of signals and thereby increases the bit error rate in a communication system. Therefore, it is needed to compensate for the above-mentioned phase offset δ, so as to correct the analog signals Sa1, Sa2 and increase the bit rate of the communication system.
There are two typical calibration methods for solving the IQ mismatch in a zero-IF receiver. One is to generate a phase difference signal by measuring the phase offset δ of the digital signals Sd1 and Sd2 and then generate a calibration signal according to the phase difference signal for compensating for the phase offsetδ of the analog signals Sa1 and Sa2. The other is also to generate a phase difference signal by measuring the phase offset δ of the digital signals Sd1 and Sd2 but generate a calibration signal according to the phase difference signal for compensating for the phase offset δ of the digital signals Sd1 and Sd2.
The above-mentioned methods both measure the phase offset δ of digital signals Sd1, Sd2 by utilizing a digital circuit of a DSP executing a Discrete Fourier Transform (DFT) on the digital signals Sd1, Sd2. Besides, phase compensation is done to the analog signals Sa1 and Sa2 by executing a Gram-Schmidt orthogonal procedure or done to the digital signals Sd1 and Sd2 by utilizing the digital circuit executing a LMS (Least-Mean-Square) algorithm. The related prior art are disclosed by the reference “Adaptive IQ mismatch cancellation for quadrature IF receiver”, Isis Mikhael, Wasfy. B. Mikhael, http://bruce.engr.ucf.edu/%7Eprp/paper6.